Contemporary semipermeable membranes are available in a variety of forms such as sheets, tubes, and hollow fibers. A "hollow fiber" is generally a hollow cylindrical structure in which the wall functions as a permeable, non-permeable, or semipermeable (i.e., selectively permeable) membrane depending upon the application. Generally, hollow fibers are used as cylindrical membranes that permit selective exchange of materials across the walls.
Liquid-separation processes utilizing membranes having selective permeabilities, such processes including ultrafiltration, microfiltration, dialysis, reverse osmosis, or the like, require a variety of materials adapted for diversified applications. For example, semipermeable membranes are currently favored for use in extracorporeal blood treatments including hemodialysis, hemofiltration, and hemodiafiltration. In such cases, the membranes typically comprise hollow fibers bundled together and assembled in a casing in a manner allowing blood to flow simultaneously in a parallel manner through the lumina of the fibers while a blood-cleansing liquid is simultaneously passed through the casing so as to bathe the exterior surfaces of the hollow fibers with the liquid.
Compounds utilized for selectively permeable membranes have included polymers, such as cellulose, cellulose acetate, polyamide, polyacrylonitrile, polyvinylalcohol, polymethyl methacrylate, polysulfone, polyolefin, or the like, depending upon the use of the membranes. Polysulfone compounds are of particular interest as they have, inter alia, excellent physical and chemical properties, such as resistance to heat, resistance to acids, resistance to alkali, and resistance to oxidation. Polysulfone compounds have been found to be biocompatible, capable of forming excellent pores and interstitia, and chemically inert to such compounds as bleach, disinfectants, and salt solutions. Polysulfone compounds can be sterilized by a number of methods, such as ethylene oxide (EtO), gamma irradiation, steam autoclave, and heated citric acid. Additionally, polysulfone compounds possess sufficient strength and resistance to wear to withstand repeated use and sterilization cycles.
Conventionally, polysulfone hollow fibers have been formed by solution-spinning techniques. Producing polysulfone hollow fibers by solution-spinning techniques typically involves dissolving a polysulfone compound in a relatively large amount of an aprotic solvent and a non-solvent, then extruding the solution through a spinneret. For solution spinning, a "solvent" is a compound in which the polysulfone compound substantially dissolves at the membrane-fabrication temperature (i.e., ambient temperature). For solution spinning, a "non-solvent" is a compound in which the polysulfone compound is substantially insoluble at the membrane-fabrication temperature. For solution-spinning techniques, the solvents must be sufficient to substantially dissolve the polysulfone compound and produce a homogeneous liquid at ambient temperature (membrane fabrication temperature).
The solvents and non-solvents utilized for solution-spinning techniques require that the membranes be extensively leached and rinsed after fabrication, as even residual amounts left in the membranes can cause unacceptable contamination of fluids treated using the membranes. Avoiding such contamination is particularly important in membranes used for the treatment of blood by dialysis or the desalination of water by reverse osmosis. When fabricating hollow-fiber membranes utilizing solution spinning techniques, removal of the core liquid used to form the fiber lumen is especially difficult. Following removal of the solvents, non-solvents, and core liquid, a non-volatile, water-soluble compound must then be added to preserve the membrane pore structure prior to drying the membrane. The non-volatile material also serves as a surfactant for later rewetting of the membranes. Such a process is known as "replasticization."
Solution-spinning techniques require the inclusion of large amounts of solvents and non-solvents many of which are generally toxic and can be difficult to extract from the resulting polysulfone fiber. Moreover, the significant amount and high level of toxicity of certain solvents and non-solvents removed from the fibers may create a hazardous waste-disposal problem.
Moreover, conventional solution-spinning techniques produce asymmetric polysulfone membranes (i.e., non-homogeneous membrane porosity progressing through the thickness dimension of the membrane). That is, a non-homogeneous membrane has a dense skin or micro-porous barrier layer on one (or both) of the major surfaces of the membrane. The dense skin or micro-porous barrier layer comprises a relatively small portion of the membrane but contributes a disproportionally large amount of control on the permeability characteristics of the membrane.
Accordingly, there is a need for a polysulfone composition and simple method for the production of polysulfone semipermeable membranes which composition and method minimizes toxic waste by-products. Additionally, there is a need for a method for the production of polysulfone semipermeable membranes wherein the solvents, non-solvents, and processing aids used in the manufacture of the membranes are easily removed from the membranes after fabrication and/or are of relatively low toxicity. There is also a need for polysulfone semipermeable membranes having a more uniform structure throughout the thickness dimension (i.e., a homogeneous polysulfone membrane) so that the entire thickness dimension controls the permeability of the membrane.